Examples of organic EL displays include passive-type ones having a simple matrix structure and active-type ones using a TFT (Thin Film Transistor).
FIG. 1 illustrates the basic pixel structure of the active type organic EL display.
A circuit corresponding to one pixel in the active type organic EL display comprises a switching TFT 101, a capacitor 102, a driving TFT 103, and an organic EL element 104.
A display signal Data (Vin) is applied to the drain of the switching TFT 101 through a display signal line 111. A selection signal SCAN is applied to the base of the switching TFT 101 through a selection signal line 112. The switching TFT 101 has its source connected to the base of the driving TFT 103 and grounded through the capacitor 102.
A driving power supply voltage Vdd is applied to the drain of the driving TFT 103 through a power supply line 113. The driving TFT 103 has its source connected to the anode of the organic EL element 104. The cathode of the organic EL element 104 is grounded.
The switching TFT 101 is subjected to ON/OFF control by the selection signal SCAN. The capacitor 102 is charged by the display signal Data (Vin) fed through the switching TFT 101 when the switching TFT 101 is turned on. The capacitor 102 holds a charging voltage when the switching TFT 101 is turned off. The driving TFT 103 supplies to the organic EL element 104 a current corresponding to the voltage, held in the capacitor 102, which is applied to the base thereof.
FIG. 2 illustrates the relationship between the display signal Data (Vin) and a light-emitting luminance (a driving current) of the organic EL element 104 in the basic pixel structure shown in FIG. 1.
In FIG. 2, RefW and RefB respectively indicate a white-side reference voltage for defining a light-emitting luminance corresponding to the white level of an input signal and a black-side reference voltage for defining a light-emitting luminance corresponding to the black level of the input signal.
In the above-mentioned active type organic EL display, a large current flows through the organic EL element 104 with respect to an image which is bright on the whole screen. When the large current flows through the organic EL element 104, power consumption is increased. When the large current continues to flow through the organic EL element 104, the degradation of the performance thereof is advanced.
Therefore, a technique for detecting the current flowing into the cathode of the organic EL element 104, and controlling the power supply voltage Vdd of the organic EL element 104 in response to the value of the detected current, thereby reducing the power supply voltage to reduce a driving current when the overall screen is bright, for example, has been developed (see JP-A-2000-267628).
The control of the power supply voltage by the above-mentioned prior art is feedback control for controlling the power supply voltage Vdd of the organic EL element 104 in response to the value of the detected current. In the case of the feedback control, when the brightness of a video is rapidly changed, for example, excessive control easily occurs. In the case, a luminance varies in a short period, which is so-called “hunting”.
An object of the present invention is to provide a luminance control method and a luminance control circuit for an organic EL display capable of achieving power saving as well as restraining the degradation in the performance of an organic EL element and capable of preventing “hunting” from occurring.
Another object of the present invention is to provide a portable telephone set capable of changing the display luminance of an organic EL display depending on peripheral brightness.
A further object of the present invention is to provide a portable telephone set capable of changing the display luminance of an organic EL display depending on the direction of a portable telephone set.